test: group examples into per-category folders

Move examples/*.sx and their expected/ snapshots into per-category
subfolders (examples/<category>/...). Folder = leading filename token,
with ffi-objc/ffi-jni kept whole; filenames are unchanged. The corpus
runner and LSP sweep now discover each category's expected/ dir, while
issues/ stays flat. Example 1058's repo-root-relative companion import
is made file-relative. Path strings embedded in 164 snapshots were
regenerated (path-only changes). Test-layout docs in CLAUDE.md updated.

examples/ffi/1214-ffi-06-callback.h

@@ -0,0 +1,13 @@
+// C-to-sx callback FFI baseline. C takes a function pointer + a value,
+// invokes the callback with the value, and returns whatever the callback
+// returned. Mirrors the `app->onInputEvent` pattern in
+// library/modules/platform/android.sx where sx installs a handler that
+// native_app_glue invokes from its input-event loop.
+
+int ffi_apply_callback(int (*cb)(int), int value);
+
+// Two-arg variant — the actual chess-on-Android shape:
+// the callback receives a pointer + a value (mirrors
+// onInputEvent(app, event) where both are opaque pointers from
+// the C caller's point of view).
+int ffi_apply_callback2(int (*cb)(void *ctx, int v), void *ctx, int v);